Conventionally, some engines have mounted thereon an exhaust gas recirculation system (EGR system) which takes out a portion of exhaust gas from an exhaust system, controls the temperature, the taking out timing, and the flow rate of the portion, and recirculates the portion to an intake system. Burnt gas contains inert gases (H2O, N2, CO2, and so on) as the main components, and when the burnt gas is put in a combustion fuel-air mixture, the highest combustion temperature of the mixture is reduced by the thermal capacity of the inert gases. Thus, the NOx contained in the exhaust gas can be reduced.
Further, an EGR system is arranged to have a flow rate control valve (EGR valve) provided in an exhaust gas recirculation path connecting an intake system and an exhaust system of an engine, open and close the EGR valve according to the operating state of the engine, and control the recirculation flow rate of the exhaust gas (EGR flow rate).
Incidentally, if the EGR valve breaks down under conditions where the valve is opened, black smoke or the like is discharged therethrough, thus reducing the commercial value of the vehicle. Therefore, in general, an automobile vehicle has a forced valve closing mechanism using a spring mounted thereon as a fail safe. Opening and closing control of the EGR valve uses a motor, and as the control algorithm therefor, PID control is frequently used, which exercises motion control over the motor by combining the controls of proportional control action, integral control action, and derivative control action.
Control actions by PID control can be expressed as a function, and the control function at that time includes control parameters, to be concrete, a proportional gain Kp, an integral gain Ki, a differential gain Kd, and a feedforward gain Kff.
There have been numerous proposals for a method concerning the above-mentioned valve control by PID control. For example, a throttle control system is known where when deviation between a target opening and an actual opening is larger than a predetermined value, the response is improved by feedforward control and further, when the deviation therebetween is smaller than a predetermined value, the stability of the throttle valve is improved by settling the valve into its normal state by usual PID control (see, e.g., Patent Document 1). Further, another throttle control system is known where its response is improved by increasing the driving force of a motor by increasing a feedforward term computed based on the change of a target opening when the change of the target opening is small (see Patent Document 2).    Patent Document 1: JP-A-1994-241098    Patent Document 2: JP-A-2000-073830
However, according to the technology disclosed in Patent Document 1, since when the deviation between the target opening and the actual opening is small, the throttle opening is controlled by usual PID control to give priority to the stability of the control over the response thereof, a delay in response deteriorates the drivability thereof. Moreover, according to the technology disclosed in Patent Document 2, a delay in response of the throttle control in the case where the target opening slowly changes is prevented, the drivability at the time of stepping on an accelerator pedal improves, and further, the throttle opening stably settles into its target opening without overshooting by controlling a feedforward term computed based on the change of the target opening, on a small scale when the change of the target opening is large. However, there is a problem that under the circumstances where the target opening is frequently changed, stable response characteristics cannot be obtained to give insufficient follow-up capability.
The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to provide a EGR valve control system where stable response characteristics can be obtained even under conditions where the target opening is frequently changed.